Method of making glass



April 28, 1936.

A. E BADGER METHOD OF MAKING GLASS Filed July 18, 1935 INVENTOR. 4. f. BADGER 9" (59 A TTORNEYS.

Patented Apr. 28, 1936 PATENT OFFICE METHOD OF MAKING GLASS Alfred E. Badger, Urban-a, Ill., assignor to Corning Glass Works, Corning, N. Y., a corporation .of New York Application July 18, 1935, Serial No. 32,063

4 Claims.

This invention relates to glass and more particularly to the method of making glasses which fuse only at very high temperatures, such as silica glass.

5 The primary object of this invention is to make transparent silica glass which is substantially free from seeds and bubbles.

When ordinary glasses are melted in the usual manner in a refractory container, a large volume of gases is evolved as a result of the decomposition of the batch materials. These gases together with air, which was entrapped in the interstices between the batch particles, form bubbles of various sizes in the molten glass. Due to the fluidity of ordinary glasses, these bubbles and seeds rise slowly through the melt and escape from the surface of the glass.

However, when the attempt is made to melt silica, that is sand, quartz, or the like, in a similar manner, entirely different conditions are encountered. Silica glass never becomes fluid as do ordinary commercial glasses but remains pasty even at 2200 C., and such high temperatures result in considerable losses of silica by. volatilization. The bubbles, which in this case consist mostly of entrapped air, will not rise through the viscous mass. The use of vacuum during melting for the preliminary removal of interstitial air followed by the application of pressure to contract any remaining seeds and cause them to disappear results in a transparent product, but the cost of production is great.

I have discovered a cheap and efficient method of producing transparent silica glass or fused silica, which comprises replacing the air between the silica particles of the batch with helium gas, fusing the silica batch to a glass containing bubbles or seeds of helium and maintaining the fused mass at a sufliciently high temperature to permit the helium to diffuse through the glass and escape.

The rates at which various gases diffuse through silica glass have been measured by several investigators. These rates of diffusion are negligible in most cases. The two gases, helium and hydrogen, are exceptional in that they possess relatively great diffusion rates. It has been found that these rates increase rapidly with rise in tem 5 perature. Furthermore, helium diffuses much more readily than does hydrogen. For example, he volume of helium diffusing through silica lass at 500 C. was found to be twenty-two times that of hydrogen. Since the helium mole- 5 cule is the smallest molecule, its passage through the network of silica glass or any silicate glass would be favored.

The diffusion rates of helium and hydrogen through silica glass have been measured up to about 900" C. and considerable resistancev is of- 5 fered to the passage of these gases even at this temperature. For example, a plate of silica 1 mm. thick and 1 sq. cm. in area would permit only 0.1 cc. of helium to pass through it per hour, the pressure of helium being one atmosphere. 10 Assuming that the rate at which diffusion increases with increase of temperature up to 900 C.

would continue up to 2000 C. or above, then the volume of gas, calculated to standard conditions, which would pass through the above silica plate 15 at 2000 C. would be about 250 cc. Although this extrapolation is probably not strictly in accordance with fact, it affords a rough estimate of the diffusion at this high temperature, which is sufilciently great to permit the manufacture of 20 transparent silica by my improved method.

In order that my invention may more readily be understood, reference is had to the accompanying drawing which illustrates'one embodiment of my invention and in which: 25

Fig. 1 is aplan view of a simple apparatus for making silica glass in accordance with my invention; and I Fig. 2 is a vertical sectional view on the line 2-2 of Fig. 1. 30

In the drawing, which represents a. molybdenum wound resistance furnace charged with a batch and ready for operation, a metal base plate II] which is mounted on suitable supports H is provided with cooling means such as a coiled tube 35 I2 through which water or other cooling fluid may be passed. Upon the base plate l0 and raised therefrom by refractory blocks I3 is placed a. refractory container I4 in which is disposed a quantity of powdered insulating material I 5 of 40 high refractoriness such as zirconium oxide. Located within the insulating material l5 and surrounded on all sides thereby is a melting container l6 which is composed of zirconia, thoria, or other super-refractory substance, and which is 45 provided with a cover l1, having a hole, and an electrical heating element I8 which is preferably of molybdenum. Within the melting container I6 is placed a batch l9 to be melted comprising silica sand, pulverized quartz or other form of 50 silica. A silica tube 20, the open end of which is immersed in the batch, extends upwardly through the hole in the cover I! and above the top of the side of the container l4 over which it is bent so as to extend downwardly and Join with an auxiliary tube 2| which is sealed through the base plate H) with wax or other suitable means.

A thermocouple 22, having its hot junction adjacent the melting container i 6 is carried through the side wall of the container I4 and extends downwardly through the base plate ill from which it is insulated by a gas tight seal of wax. In a similar manner a pair of copper lead-in wires 23 is sealed through the base plate I0 and extend upwardly and over the edge of the container H to join with the ends of the heating element l8. Inlet and outlet tubes 24 and 25 respectively are also sealed through the base plate H]. A bell jar 26, which preferably is composed of metal such as iron but may be composed of any heat resisting gas tight material such as glass, rests upon the extremities of the base plate ill to which it is sealed with a wax seal 21 and serves to exclude air from the assembled furnace and its contents and to maintain within it the special at:

' the outlet tube 25 and maintain within the apparatus an atmosphere of hydrogen in the manner customary for the operation of molybdenum wound furnaces. When the above recited conditions are attained an electric current is applied to the lead-in wires 23 so as to heat the molybdenum element l8 and fuse the batch I9. When the batch fuses, the portion of the tube 20 which is immersed therein will likewise fuse and the tube will be severed above the glass line. The incoming stream of helium may be either continued or discontinued, or it maybe replaced by a stream of any other gas which is inactive toward molten silica and highly heated molybdenum. The fused charge of silica containing bubbles of helium is then maintained in the molten condition until the helium in the bubbles has diffused through the viscous mass and escaped leaving the melt substantially free from bubbles. During the diffusing step it is preferable to discontinue the passage of helium through the tube 2|, both on account of economy and also because diflusion from the bubbles takes place more rapidly if a low partial pressure of helium is maintained in the atmosphere surrounding the melt. The temperature of the molten mass may readily be'ascertained for ease 01' control by means of the thermocouple 22 which is used in the customary manner in conjunction with a potentiometer or galvanometer, not shown.

After being melted and fined as described above, the transparent silica glass may be removed from the melting container and be reheated and molded or drawn in the manner known in the art of fabricating articles of silica glass.

While in the above specification I have disclosed my invention as applied to the clearing of glass resulting from the fusion of substantially pure silica, it is also applicable to the clarification of other glasses which are permeable to helium when heated and hence in the following claims I will use the term "glass as a generic expression including silica glass or fused silica.

What I claim is:

l. The method of making glass which is substantially free from bubbles which includes fusing the batch therefor in the presence of helium so that the bubbles which remain in the fused glass and which would normally contain air and other gases, will instead contain substantially pure helium and maintaining the glass in fused condition until the helium has difiused out of the bubbles.

2. The method of making transparent silica glass, which includes replacing the interstitial air in the batch with helium, fusing the batch to produce a molten mass of silica having bubbles which contain helium and maintaining the silica at high temperature until the helium has diffused from the bubbles.

3. The method of making transparent silica glass, which includes passing helium through the batch to replace the interstitial air therein, fusing the batch to produce a molten mass of silica having bubbles which contain helium and maintaining the silica at high temperature in an atmosphere which is substantially free from helium until the helium has difiused from the bubbles.

4. The method of making transparent silica glass, which includes passing helium through the batch to replace the interstitial air therein, fusing the batch to produce a molten mass of silica having bubbles which contain helium and maintain- 

